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Thursday, June 18, 2009

In a recent New Scientist, (May 30, 2009 p8) it was suggested that the human induced demise of fish stocks, both fresh water and oceanic, began long before the modern era. Many lines of enquiry are quoted to support this contention. This may well be so and would parallel the destruction of populations of land animals as humans invaded each new area. Be that as it may, modern humans with their technology have finished off the job quite nicely and we are now in the situation in the oceans that Southern Africa was before the whites woke up to the fact that most of the animals of their forefathers were just about to go extinct. They took the appropriate measures and at least, in reserves which they set aside, the fauna of Africa recovered. (it is now on the way out again)

With respect to the oceans, lets consider the problem from a different perspective; to explore a different way of thinking. We have, at a conservative estimate, destroyed at least 70% of the fish resources of the world that existed at the start of the modern era and in the case of, for instance, the Grand Banks off Newfound Land, even though fishing has been banned for decades, the fish resources are not recovering. Lets look at the problem from the point of view of primary production; from the point of view of phytoplankton.

Ultimately, the productivity of 99.9%+ of biological systems on earth is based on photosynthesis. The ultimate limit to productivity depends on how much sun energy falls on the system. This is something which we can not increase Ultimately the limit to primary production is sunshine. The more of this sunshine we can absorb by building simple mollecules into more complex mollecules throught he agency of life, the greater the productivity of a system.

There is a principle in the growth of individual animals and plants and in the productivity of ecological systems that says that growth/increase/primary productivity depends on the most limiting factor; that is to say the factor which is in shortest supply in comparison with the amount which would not limit growth. In an ocean system, water is not limited and sunshine is whatever nature provides so the main limiting factor is the availability of nutrients. And the potential for production is enormous.

Look at, for instance, the water off Peru in non-El Ninio years. In these years there is upwelling of nutrient rich waters from the deep ocean. We don't actually see primary production (phytoplankton) or even secondary production (zoo plankton) but only tertiary production which in Peru takes the form of Anchovies. The fisheries is humongous and provides much of the fish meal for the world from this one small patch of ocean. This fish meal is used in feed for most land based domestic animals and in the huge fish farming (feed lot) industry.

Another relevant fact is that only 10% of the material goes from one tropic level to the next. A hundred kg of phytoplankton will make 10kg of zoo plankton and 10 kg of zoo plankton will make 1kg of anchovy. The primary production off Peru must therefore be about 100 times as large as the Anchovy production. Lesson:-- Potential primary production in the oceans is very very large.

So how does this relate to our subject. Lets do a thought exercise with a simple system consisting of phytoplankton, krill, penguin, leopard seal and killer whale. Each feeds on the layer below it. OK so the killer whales sometimes take penguins but lets keep it simple. We'll assume a moderate level of nutrient input from upwelling sea water. Oh and we will need a population of bacteria to recycle carcases and the poop of these various animals.

We start with the water full of nutrients and inoculate with phytoplankton. The phytoplankton starts to grow explosively and remember some algae can double every hour when the sun shines. Sunshine in the Antarctic summer is 24 hours a day. Raise 2 to the 24th power for one day and then add on a couple more days and pretty soon you have masses of algae. The nutrients are quickly used up including the small amount being added and the algae become senescent (old and dying). Primary production slows and dead algae begin to sink to the bottom. A small amount of mineralization by bacteria returns some nutrients to the system. Primary production ticks along at a much reduced level limited by the influx of new nutrients and some bacterial mineralization.

Incidentally, in systems such as coral reefs and tropical jungles the input of nutrients is very low and the whole, incredibly rich system only remains vibrant due to the very tight circulation of nutrients within the system. More on this later.

Drop in some krill. The krill start to eat the phytoplankton. Now remember that only 10% of the eaten phytoplankton becomes krill. The rest is pooped out into the water. This 90% is phytoplankton-nutrient. Even better, not all phytoplankton need completely mineralized nutrients (broken right down into phosphates nitrates and other 'ates') but can use higher molecules much as bacteria do. And you can depend on it that with all these more energetic molecules

The penguins eat krill and poop out nutrients. As with the krill, 90% of what they eat becomes available for the algae. By suppressing the population of krill they delay or might even stop the krill from crashing the system. Unlikely. The system is still too simple. Algae production increases once more. Penguins live longer than krill and boom and bust at a slower rate. Having a longer cycle also avoids resonance which can occur if the eater and the eaten have the same length cycle or a cycle which is a multiple of each other. Note that if there is a standing crop of a million kg of krill, there is likely to be a tenth or less that this of penguins. Less and less biomass as you go up the ladder. The surprise here is that over the long term, there will be more biomass of krill than there was without the penguins and far more total biomass. More of the available sun energy, which is the ultimate limiting factor, will be used.

Add in the leopard seals which eat penguins and then the killer whales which eat the leopard seals. Each layer is smaller in terms of kilograms than the one before it, each layer poops out (cycles) nutrients for the use of the phytoplankton. Each layer improves stability. Each layer increases total primary production. If the nutrient cycling is very tight (not much exiting the system by, for instance, falling to the bottom and becoming buried), you can get an extremely rich environment such as exists in a tropical forest or a coral reef.

If on the other hand we have a system in which all the nutrients except what is returned by old age, is trapped in a given level, then primary productivity stagnates. It is like money in an economy. It only does its job if it is circulation. If money is put in a bank, it is invested and it continues to work. If it is put under a mattress, it stops enabling the system. The total biomass that can be supported in the system increases as you add an extra layer and by more than just the amount added by that layer. In other words, as you add the leopard seals, the total biomass of algae, krill and penguin increases. Of most importance, the total amount of sun energy which is being captured increases.

This might be the solution to such mysteries as why the grand banks are not recovering as expected. I can think of a few other contributors to that problem but as the cod are removed, which are a third or fourth level predator, nutrients are not being recycled nearly as quickly and primary production is reduced. One wonders what is the effect of the demise of the whales. Remember that some of them, after feeding in the rich arctic or Antarctic oceans, traverse less productive areas and even though they will soon cease to defecate, they will continue to urinate as they use up their stored energy. In the polar regions, they feed at the same level as the penguins and so recycle nutrients very quickly. Some whales hunt at depth and poop on the surface; a biological upwelling system. What loss to total primary productivity is due to them no longer being extant. And similarly what was the contribution to their environment of the huge schools of tuna that once cruised the oceans.

Going back to our example, if we eliminated all the penguins, leopard seals and killer whales, we would look at the system and see a greatly reduced krill population. We would make our calculations and surmise that we could only support a relatively small population of penguins if we returned them to the system. We forget the lesson of the tropical jungles and the coral reefs. If a system can circulate its nutrients within itself, it can support a huge population on very small net inputs. As long as nutrients are recycled to the photo synthesizers, huge primary production can occur with huge biomass. The populations of lower levels in the chain depend on higher levels keeping nutrients in circulation.

This brings us to how much we can harvest. If we note that despite the best efforts of the phytoplankton, there is an excess of nutrients in the water, we may be able to mine the system to an extent. As we do, we are removing nutrients from the system like a farmer who takes a sheep or a cow off a field and sells it. We will reach a point where there are just enough nutrients to allow unlimited growth of the photosynthesizers. At that point we can only harvest whatever crop we are after at the rate that nutrients are being added from the outside to the system. In the case of our farmer, this is at the rate that he adds fertilizer back on to his field. In the case of the ocean, it is at the rate of replenishment by upwelling (biological or pysical). If we deplete the system so that photosynthesis is not running at the "sun limit" then we reduce the productivity of the system and hence the amount we can harvest sustainably. In the long term, you can only harvest a system at the rate at which nutrients are being added to the system. Mining a system (taking more than the input) will eventually crash the system.